perf unwind: Report error from dwfl_attach_state

perf unwind: Report error from dwfl_attach_state

In verbose level 2, errors returned by libdw are reported in most cases,
but not when calling dwfl_attach_state.

Since elfutils v 0.160 (2014), dwfl_attach_state sets the error code to
report failure cause. On failure, log the reported error.

Signed-off-by: Martin Vuille <jpmv27@aim.com>
Reviewed-by: Kim Phillips <kim.phillips@arm.com>
Link: http://lkml.kernel.org/r/20180318175053.4222-1-jpmv27@aim.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>